Why you would want to build your own breathalyzer? Well, everyone has their own reasons I guess. For us, it’s enough to know it can be done. It’s not precise, but will give you comparison values (“I’m drunker than you are”. “No, I’m drunker”). I don’t recommend this as a reason for drinking games, but it is an interesting application of technology.
http://www.danielandrade.net/2010/03/07/building-an-breathalyzer-with-mq-3-and-arduino/